luiginumberone

Best answer as chosen by Asker / Ratings
It's kind of hard to tell you where to look if we don't know which ones you have, which is pretty much impossible to remember. The best possible help is to simply go through the mansion room by room one more time.

However, there are a number of secret rooms, such as the treasure room through the mouse hole in the Butler's Room, and the boarded up room accessible from the roof. I believe there is a boo in either or both of those, so check those. If you don't know how to get into them, check a walkthrough.

pity

Enter the ghost girl's room or the treasure room(both can be accesed from the mansion's roof) I'm almost completely sure that it is hidden there. Hope this helps and good luck!